(1) Field of the Invention
The present invention relates to a technique for monitoring optical dispersion based on waveforms of transmitted light. In particular, the present invention relates to an optical dispersion monitoring apparatus and an optical dispersion monitoring method, capable of monitoring dispersion accurately with a simple construction, and to an optical transmission system using the same.
(2) Description of the Prior Art
In optical communication, as shown at the upper part of FIG. 17 for example, an optical signal sent to a transmission path 101 by an optical transmission apparatus 100 is transmitted for several tens of kilometers to several thousands of kilometers through an optical fiber via optical repeaters 102 using optical amplifiers or signal regenerators, to be received by an optical receiving apparatus 103. At this time, waveform distortion occurs in the optical signal being transmitted, due to nonlinear optical phenomena occurring in the optical fiber depending on a dispersion characteristic of the optical fiber or the intensity of the optical signal, a change in instantaneous optical frequency of a pulse added in the optical transmission apparatus 100, and the like.
To be specific, in the case where a single optical pulse is transmitted through a long distance optical fiber for example, depending on the wavelength of the optical pulse or the characteristic of the optical fiber, “pulse compression” in which the pulse width is narrowed and the peak power is increased, or “pulse spread” in which, conversely, the pulse width is spread and the peak power is reduced, occurs as shown in FIG. 18. Such waveform distortion of optical pulse causes signal interference between adjacent bits in the data transmission, and is therefore a significant problem.
In order to cope with the above described problem, in a conventional optical transmission system, as shown at the lower part of FIG. 17 for example, there is known a structure in which dispersion compensators 104 are inserted in the transmission path at appropriate spacing to compensate for cumulative dispersion, so that a dispersion characteristic of the whole system is in an optimal condition. Furthermore, in an optical transmission system actually operated, since the dispersion characteristic of optical fiber varies over time, sometimes just a single variable dispersion compensator dynamically compensating for variation over time may be used on its own, or in combination with a fixed dispersion compensator performing a large amount of dispersion compensation. The lower part of FIG. 17 shows an example in which a variable dispersion compensator 104A and a fixed variable compensator 104B are connected in series to construct a dispersion compensator 104. In order to operate the variable dispersion compensator 104A to perform the dynamic dispersion compensation as described above, an optical dispersion monitoring apparatus 105 is required for determining whether a compensation amount is optimal or not, while the system is operating.
For a conventional optical dispersion monitoring apparatus, there is for example a structure in which cumulative dispersion is detected by paying attention to the spectral shape or spectral intensity at a specific frequency of a received optical signal. Furthermore, there is also known a structure in which the error rate of a regenerated signal at a required monitoring location is measured to detect cumulative dispersion.
Moreover, in Japanese Unexamined Patent Publication No. 2001-320329, a technique is proposed in which a received optical pulse signal is converted into an electrical pulse signal, and depending on the voltage level obtained by rectifying and smoothing an AC component of the electrical pulse signal, it is detected whether the occurred waveform distortion is the pulse compression or the pulse spread.
However, the following problems arise in the conventional optical dispersion monitoring apparatus as described above. Namely, in the system for paying attention to the spectrum of received optical signal, a significantly high quality device is required, since the spectral intensity at a specific frequency is extremely low, and the spectral intensity is easily influenced by frequency characteristics of optical filters, light receiving elements, monitoring circuits, and the like. Consequently, there is a problem in that it is difficult to easily realize an optical dispersion monitoring apparatus.
Furthermore, in the system for measuring the error rate, there is a drawback in that even if it is possible to detect the existence of cumulative dispersion based on the measured error rate to detect an absolute value of cumulative dispersion, the sign of the cumulative dispersion cannot be extracted. In addition, since a signal regenerator is required to measure the error rate, there is a problem in that the locations where an optical dispersion monitoring apparatus can be installed are limited.
Moreover, in the technique proposed in Japanese Unexamined Patent Publication No. 2001-320329, since the construction is such that the occurrence of waveform distortion is detected depending on the temporal average power of a mark component of a received optical signal, it is possible to detect whether the waveform distortion is the pulse compression or pulse spread, however, there is a problem in that it is difficult to detect the cumulative dispersion including sign information with high accuracy.